1. Field of the Invention
The present invention relates to reflector type antennas and more particularly to doubly reflective antennas capable of rapid mechanical beam scanning.
2. Description of the Prior Art
Antennas capable of rapid mechanical beam scanning with the minimum of mechanical complexity while maintaining critical beam parameters throughout the scan limits was described in a paper entitled A Rapid Wide Angle Scanning Antenna with Minimum Beam Distortion, delivered by Martin and Swartzman at the Fifth Annual East Coast Conference on Aeronautical and Navigational Electronics in October 1958. Antennas of the type disclosed by Martin and Swartzman utilize an initially illuminated reflector constructed of grids parallel to the polarization vectors of the incident energy. Beams incident to the grided reflector are reflected therefrom to a flat deflecting plate pivotal about the antenna feed. This pivotal plate is constructed to provide a deflected beam having a polarization perpendicular to that of the polarization of the beam incident thereto. These polarization twisted signals propagate through the grided initial reflector into free space at an angle from the antenna axis that is determined by the angular position of the pivotal plate. Thus rapid mechanical scanning over wide scan angles with a minimum of beam distortion is provided. These antennas, however, are polarization sensitive, being operable only over a small range of polarization angles.
Applications exists that require a rapidly scanning dual polarized antenna capable of operating at both polarizations simultaneously. Many of these applications exhibit antenna requirements that vary with scan angle. During the search mode, the antenna of a guided missile seeker is positioned at a steep look-down angle (the angle of the radiation axis relative to the missile axis) and maximum system sensitivity is required, since the missile is at a great distance from the target.
Since signals reflected from targets illuminated at one polarization possess strong cross polarized components, to achieve this maximum sensitivity the antenna must receive both polarizations. During the track mode, the look-down angle is appreciably reduced and the missile is relatively close to the target permitting reception at a single polarization for reduced system sensitivity is acceptable.